It has been proposed that a beta-type zeolite obtained by being subjected to ionic exchange with iron ions is used as a vehicle exhaust gas purification catalyst (See, Patent Documents 1 to 3). In Patent Document 1, for example, a denitration catalyst comprising a carrier obtained by a beta-type zeolite having an SiO2/Al2O3 molar ratio of 15 to 300 being subjected to ionic exchange with 0.1 to 15% by mass of Fe3+ ions, and ferric oxide carried on the carrier is described.
In Patent Document 2, it is described that a beta-type zeolite having a skeletal structure in which Si percentage content assigned to Q4 of zeolite skeleton as observed in 29Si MAS NMR spectrum is 35-47% by mass and having an SiO2/Al2O3 molar ratio of equal to or more than 20 but less than 100 is subjected to ionic exchange to cause it to carry Fe3+ and to be brought into contact with exhaust gas containing nitrogen oxides.
In Patent Document 3, a method for producing an NOx adsorbing material is described. This method comprises an impregnating step of impregnating a beta-type zeolite with iron chloride aqueous solution to form an iron chloride-containing zeolite, an ionic exchange step of heating the iron chloride-containing zeolite at 330-500° C. under an atmosphere containing no water to cause Fe to be subjected to ionic exchange, and a heat treatment step of heat treating the iron chloride-containing zeolite after the ionic exchange step under a non-oxidizing atmosphere.
Patent Document 1: Pamphlet of PCT International Publication No. WO2006/11575
Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2007-076990
Patent Document 3: Japanese Unexamined Patent Application, Publication No. 2008-264702